1. Field of the Invention
The invention relates to a fermenter-decanter intended for the purification of residual waters, particularly from sugar refineries, with recovery of the fermentation gases which consist for the most part of combustible methane.
2. Description of the Prior Art
Conventional purification by storing the residual water from sugar refineries in static water, this residual water containing various sugars which are destroyed by fermentation during the storage in the static water, requires very large tanks, causes the release of disagreeable odours into the environment, does not permit precise control of the effective purification level and, finally, means that the energy contained in the sugar products is left to disperse into the ambient medium.
The inventor and his fellow-workers have made studies in the purification of residual water by fermentation under the effect of mesophilic anaerobic micro-organisms (i.e. those which are active in a temperature range around 35.degree. C.), the results of this type of fermentation being mainly methane and carbon dioxide, the latter being largely held in a carbonate combination, whilst the methane which is released constitutes a useful fuel. In fact, fermentation occurs in two stages; in the first stage, acidifying micro-organisms convert the sugars into organic acids, whilst strictly anaerobic mesophilic micro-organisms decompose the acids into methane, carbon dioxide and water.
The inventor's studies were based on laboratory tests, then tests on pilot fermenters. A first pilot plant, comprising a 90 m.sup.3 tank, at the Escaudoeuvres sugar refinery, was followed by a second with a 1580 m.sup.3 tank at the sugar refinery of Vauciennes. A description of the work and the plants will be found in the journal Sucrerie Francaise: "La depollution des eaux", by J. P. Lescure and P. Bourlet, March 1977, pages 103-109; "Nouvelles perspectives pour le traitement des eaux en sucrerie; la fermentation methanique mesophile", P. Devillers, J. P. Lescure and P. Bourlet, April 1977, pages 173-183; and "Traitement des eaux residuaires par fermentation methanique mesophile", J. P. Lescure, P. Bourlet, March 1978, pages 107-114. In the plants described, fermentation took place at about 35.degree. C. by the passage of the affluent into a heater, from which this affluent was carried along piping to the bottom and near the centre of the tank, where it was mixed up by a rotary agitator. A cover made of reinforced butyl rubber was anchored by its periphery and submerged in the tank and covered the fermentation zone so as to form a collection pocket for the methane released, which was evacuated via tubing fixed to the upper part of the cover pocket. This arrangement, which resulted in rapid completion of the fermentation process, nevertheless had some disadvantages which affected the safety of use and the reliability of the installation. In fact, the cover was not tensioned and partially emerged from the water. The agitation of the residual water below the cover was transmitted to the cover, which then became subject to premature cracking. Leakages through the cover, through the beginnings of cracks, created a danger of explosions or fire in the pocket inflated by the gas.